Question answered
Torching under stone slates
The short answer
Torching under stone slates is a layer of lime mortar pointed or pushed into the gaps on the underside of the roof, visible from the loft. It helps hold slates steady and keeps wind-blown rain and fine snow out of the roof space. On historic roofs it is usually kept or renewed in lime, and on a listed building a change of method can need consent.
Torching is the lime finish sometimes found on the underside of old stone slates. It is easy to overlook from the street, yet it affects how the roof sheds wind and what repairs will suit it.
What torching is and where to find it
Climb into the loft of an old stone roof and look at the underside of the slates. In some roofs the gaps between them and the laths are filled with a grey or off-white mortar. That is torching, the name for lime mortar, often with fibre in it, applied from inside the roof.
It is separate from the weathering of the roof itself. The stone does that from the outside. Torching acts as a seal and a bed from within, closing the small gaps that irregular slabs always leave between one another.
Not every stone roof has it, and the method varies from roof to roof and from one period to another. Do not assume yours does, or does not, until someone has looked. The Stone Roofing Association's guide to Horsham roofs is the best place to start if you want the wider picture of how they are built.
Why lime and not a modern mix
Lime is softer and more porous than cement. It lets a damp roof breathe, it moves a little with the timbers and it can be taken off without damaging the slate or the frame. Cement is hard, traps moisture and can crack away, sometimes pulling at the slates as it goes.
That is why conservation advice leans toward lime on old buildings. Horsham District Council's Steyning appraisal criticises laying slab roofs with mortared joints, which shows how seriously the character of those joints is taken. Mortar in the wrong place can spoil appearance and behaviour alike.
Mixes vary with the roof, so there is no single recipe to quote. We match what is there where it is sound, and talk through the choices with you before touching it, because lime that has been in place for generations deserves a reason before it is disturbed. Photographs from the loft, taken before and after, give both you and any conservation officer a record of what was done. They also help the next owner, who will otherwise be left guessing which patches are original and which were added later.
Signs that torching has failed
From the loft, look for finish that has dropped away in lumps and lies as grit on the ceiling boards or insulation. Look for bare gaps where daylight shows, and for slates that have started to shift because their bed has gone.
On a windy day you may feel a draught through the underside, and after storms you may find dust or a fine scatter of snow in the roof space. Those point to openings where the finish used to be.
Failed torching is rarely the only fault. Slipped slabs and rotted laths often come with it, so the whole underside deserves a look, not just the worst patch. How long stone roof repairs take shows how a job is sequenced once the loft has been inspected, and the main Horsham stone roofing page sets the scene.
Renewing it properly
Renewal is easiest when the roof is being relaid, because the slates are off and the underside lies open. In that case the lime finish can be put back as part of the job. Doing it in place from the loft is possible, but slower and more awkward, and the result depends on how well the old surface can be cleaned.
Either way, the lime must go onto sound laths and a clean surface, and it must not block ventilation or trap damp against timber. A roofer who tells you cement will do is not following the approach conservation officers expect.
Single lap roofs have their own detail, in which small Welsh slates called shadows, bedded in mortar, cover the slab joints; single lap Horsham stone explains it. The finish inside and the shadows outside should be treated as one system, and one person should be responsible for both.
Consent and approvals
On a listed building, the Adur and Worthing councils expect listed building consent when a repair departs from the original materials, while the National Park Authority treats small repairs done like for like as outside it. Whether renewing torching in lime counts as upkeep or as a change is for the local authority to say, so ask. Stone roofs in the South Downs National Park shows who to ask there.
Building Regulations also come into play if a relay changes the covering or its weight. See Building Regulations when changing a stone roof.
Our survey includes the underside wherever we can reach it, and the photographs go to you with the written quote, which sets out the whole job as one price. If the loft is cluttered, clearing a path to the eaves beforehand lets us see far more of the finish, and that usually makes the advice firmer and the quote easier to stand behind.
Torching: two quick questions
Can I add torching to a stone roof that never had it?
It is possible, but ask first. On a listed building that is a change to the fabric and may need consent. A survey will show whether the roof has a reason to want it, such as persistent draughts or driven rain entering the loft.
Is crumbling torching an emergency?
Usually not on its own. It does mean the roof is less sealed, so deal with it before winter and look for slipped slates at the same time. If rain is already getting through, tell us and we will cover it temporarily.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.